A recurrent handicap of the methods based on molecular (or homogeneous) catalysis lies in the difficulty in separating the products and the catalyst in order to recycle the latter. In most cases, these methods work on a lost catalyst basis with, in addition, the potential problem of toxicity of the catalytic species present, even as traces, in the products. “Heterogenization” of homogeneous catalysts is thus a problem widely studied. One possible way consists in immobilizing the catalytic species on or in a support, notably mineral.
The article by Zhao et al. (Materials Today, 9, 3, p. 32-39, 2006) reviews several molecular catalyst and enzyme immobilization modes, and it notably mentions chemical grafting, electrostatic grafting, adsorption and encapsulation. Immobilization by adsorption is essentially based on the Van der Waals forces alone, and it is notably very frequently used to immobilize enzymes in ordered mesoporous materials. In fact, this type of immobilization is simple and requires no treatment of the support, which allows not to denature the enzyme.
The worldwide demand for alpha-olefins, intermediaries used in the manufacture of surfactants, additives for lubricants and fuel bases, continues to grow. There are several commercial methods of oligomerizing ethylene to hexene-1 by molecular catalysis and a considerable amount of work is available today on the modification of these systems in order to direct them towards the selective production of octene-1 or higher oligomers.
The article by Deckers et al. (Angew. Chem. Int. Ed. 2001, 40, 2516-2519) presents an organometallic complex wherein the Ti+ ion is coordinated by a hemilabile ligand η5-C5H4CMe2C6H5 and used to selectively produce hexene-1, the co-catalyst used being methylaluminoxane (MAO). The reaction flowsheet corresponding to the trimerization of ethylene in a method using this organometallic complex as the catalyst is detailed in the article by T. de Bruin et al. (Organometallics, 2003, 22, 3404-3413). The mechanism that seems to be well established today involves a succession of metallacycle intermediates and of transition states. For each metallacycle intermediate obtained upon ethylene oligomerization, there is either a possibility of cycle opening through hydrogen transfer, subsequently leading to the desorption of the corresponding alpha-olefin (butene-1, hexene-1, octene-1 or decene-1, respectively for 2, 3, 4 and 5 ethylene molecules strung together), or a possibility of growth of the metallacycle intermediate by two additional links through the insertion of a new ethylene molecule.
It is of particular interest to be able to influence the reactivity by acting on the activity and the selectivity of the catalytic system in order to selectively obtain an alpha-olefin such as, for example, octene-1 or decene-1. The present invention, which furthermore allows easy separation for recycle of at least part of the catalytic system, falls within this scope.